Update builders documentation

[rust1248]

Fixes #266

Update the Builders documentation to make clearer how to construct them

[sfackler]

There are some rustfmt failures.

[rust1248]

These are not mine (I only updated the docs)

[sfackler]

The rustfmt failures are pointing at the docs you updated:

 }
 
 /// A builder for `TlsConnector`s.
-/// 
+///
 /// You can get one from [`TlsConnector::builder()`](TlsConnector::builder)
 pub struct TlsConnectorBuilder {
     identity: Option<Identity>,
Diff in /home/runner/work/rust-native-tls/rust-native-tls/src/lib.rs at line 5[19](https://github.com/sfackler/rust-native-tls/actions/runs/5342182950/jobs/9726590832?pr=271#step:4:21):
 }
 
 /// A builder for `TlsAcceptor`s.
-/// 
+///
 /// You can get one from [`TlsAcceptor::builder()`](TlsAcceptor::builder)
 pub struct TlsAcceptorBuilder {
     identity: Identity,

[rust1248]

@sfackler Sorry for that, I only saw the other error

I fixed them (I think)